This started happening out of the blue, after driving my truck a bit hard without giving it time to warm up (I'm not sure that has anything to do with this).

It idles very rough with very little power when driving - it can rev high but there just isn't power to accelerate. I can get to speed but it takes a lot of slow acceleration. It dies sometimes when I'm shifting or slowing to turn a corner and I push the clutch in, and has trouble starting sometimes. When I'm pushing it hard, I smell what smells like 2 stroke exhaust (weed eater).

I thought I found it when I found a gray vacuum line broken that goes to the heater core switch, but I would expect changing the AC settings would stop that vacuum leak from affecting the engine. Am I wrong? Pinching the end of the broken vacuum line doesn't affect the idle.

Other things I've tried, so far:
* No codes on ECU, no CEL - I have a chinese ripoff OBDII reader that can pull codes and reset them when I've had issues in the past, but in this case idling for long periods and driving for short distances doesn't produce new codes. I did notice that the oxygen sensor 2 is showing 0V while I monitor it with my OBDII app (Torque). I don't trust that reading, since I assume a reading like that would trip a code. Am I wrong? Also, do cheap OBDII readers not read all codes?
* Test coil packs - proper resistance between them, and when I pull the spark plug wire off while it's running I see sparks arcing.
* Checked spark plugs - all 8 (& wires) replaced 3 months ago. I pulled a few and they all looked pretty good with proper gaps. A little sooty and 1 had a bit of oil
* Checked voltage to MAF - gets good battery voltage and idling the sensor sends about 1.5V and revving it gets up to 2.25V or so. It was tricky getting the reading because i didn't have good probes and had to grab the throttle cable while holding the probes. When I pull the MAF cables out while it's running, the idle doesn't change much if at all - does that mean it's a bad MAF (as this video claims? youtube.com/watch?v=b4U_CGkS8ag) - I couldn't pull off the MAF to check if it's dirty/clean it because I don't have the security torx head
* Cleaned IAC - I pulled it off and cleaned it with cleaner, but it wasn't particularly dirty. The grommet/pad was brittle and disintegrated, so when I put it back on it's now just metal on metal with no discernible change in idle
* New PCV hose - This was a long shot, but the old one was soft and falling apart, so I put a new one on. No change
* Incompetent vacuum testing - I sprayed around some carb cleaner listening for a change in idle, but I don't know where all the vacuum lines are.
* Fuel Pressure Check - I checked the fuel pressure with a rented gauge, and got 64 PSI while idling, which is within spec according to my Haynes manual
* Seafoam - I put in a healthy amount of seafoam just in case it was something easy like that. No change

That's outside my comfort zone but I'll give it a shot. I suspect the cat is clogged, whether that's a symptom or the original cause, it seems like it's definitely affecting it now, since the performance while driving it around has steadily gotten worse. Tomorrow I'm going to try and pull the cat off and see if that fixes it. Any other obvious way to diagnose a clogged cat?

And just for shits and giggles.. when you pulled the connector on the MAF, did you drive it or just tug on the T/C with little difference in "sound & idle"...?

And you could just break the Catalytic bolts lose at the front flange, and drop or bend the exhaust just a little out of the way. As long as exhaust can escape, should be enough to uncork it and breath...

And just for shits and giggles.. when you pulled the connector on the MAF, did you drive it or just tug on the T/C with little difference in "sound & idle"...?

And you could just break the Catalytic bolts lose at the front flange, and drop or bend the exhaust just a little out of the way. As long as exhaust can escape, should be enough to uncork it and breath...

Those bolts were on tighter than the o2 sensor, which I finally got off by heating the exhaust, hitting the base of the sensor with a rod and hammer, and using a pipe. I ran it a bit without the o2 sensor, and loads of exhaust was coming out of that hole. That doesn't necessarily mean the cat is clogged though, since it is just the path of least resistance...

It seemed to idle more regularly/smoothly, aside from the louder noise, but there was absolutely no power. I backed out of my driveway to take it around the block, but realized I had no power and could barely get it up into the driveway again. I really wish it was the cat because I'm so sick of troubleshooting this.

I also got the right tool to pull off and inspect the MAF elements - they look fine/clean.

BTW, while I was letting the engine warm up the exhaust manifold before I got the sensor off, I noticed exhaust drifting around that spot, near the flange. I couldn't track down where it was coming from, but it doesn't seem to belong there.

I did a power balance test as described here (https://www.ericthecarguy.com/faq/so...alance-testing). I pulled the spark plug wires off the left side, from rear to front, and no change in RPM until I got to the front plug. When I pulled that, the truck died. Sounds like I'm running on 1 cylinder?

I'm planning on doing a compression test this morning.

Edit: I performed a compression test on the left side. I removed the spark plugs and plugged in the tester I rented from Autozone. According to the tester, none of the cylinders are holding pressure, and since I was doing it myself I couldn't see the PSI reached. The fact that it's not holding pressure in ANY cylinder is concerning to me. I thought that at the very least the first cylinder would keep pressure. I am about to give up on this truck.

This started happening out of the blue, after driving my truck a bit hard without giving it time to warm up (I'm not sure that has anything to do with this).

*Do you see something obvious that I missed?*

Brad

Not that 200K is not a lot of miles, it certainly is not a young and fresh vehicle. But with care they can last twice that and still take the rambunctious romp down the highway or trail. However, things get worn down, often pre-maintenance is a far thought unless a problem goes from bad to worse. There are countless variables in the actions in your own statement to be observed..

Since it is an inline4, you could invariably pull the head and pan and check the condition of the engine, pistons, cylinder walls, and ruminants in the oil pan itself and see where you're positioned. It sounds like you have some mechanical sense and drive, with available funding. You could find yourself needing a quick hone, a replacement and clocking of some new rings or a head gasket and be on your way.

Then again, you could have scored the cylinder walls bad enough to be beyond machining and have to swap an engine. Either way, you will know once you get the head off.

Thanks for the help, both of you. I gave up and washed my hands of this, selling for $400 to a guy who drove it away on 1 cylinder. Godspeed, mystery man. It kind of hurts to give up on it, but it was only worth $1000 running (lots of cheap trucks in this market).